System for investigating density fields



Sept. 23, 1941. T. ZOBEL 2,256,855

SYSTEM FOR INVESTIGATING DENSITY FIELDS Filed Nov. 7, 1939 PatentedSept. 23, 1941 SYSTEM FOR INVESTIGATING DENSITY FIELDS Theodor Zobel,Brunswick-Lehndorf, Germany,

assignor to Luftfahrtforschungsanstalt Hermann Giiring, Brunswick,Germany Application November 7, 1939, Serial No. 303,331 In GermanyNovember 8, 1938 Claims.

This invention relates to optical arrangements and methods for examiningthe density of materials and test mediums in a qualitative andquantitative respect.

There are quite a number of physical processes in which a change of thedensity of transparent or translucent bodies can be rendered visiblewith the aid of the striae method of Toepler. method briefly statedconsists in that a bundle of light rays is passed through the densityfield or test medium to be examined and then directed upon a screenthrough an optical system including a condenser lens or a concavemirror, a stop member in the form of an edge being arranged at the focalpoint of the rays, so as to be just in contact with the bundle of raysat this'point provided the rays pass through a medium having normaldensity, such as air. Now, if the density of the medium is differentfrom that normal value, the bundle of light rays will be somewhatdeflected, towards or away from the stop edge, depending on therefraction of ,the medium to be investigated, and the image of the lightrays on the screen is thereby brightened or darkened, respectively. Thismethod in many instances is satisfactory for a qualitative observationof the phenomena but, it does not permit -a quantitative investigationof the phenomena to be examined. On the other hand, as is well known,the interference method in its various modifications is very suitablefor such quantitative examinations. The interference method, brieflystated, consists in that a bundle of light rays is split up into twopartial bundles of rays one of which passes through the medium to beinvestigated while the other ray is made to travel through a parallelpath of exactly the same length, but without any test medium beingarranged in this parallel path; the two partial rays are then re-unitedand made visible on a common screen. By suitable adjustment of mirrorsin the light paths, it is now possible to produce interference stripsthe width and shape of which depend upon the density of the test medium'and form a direct quantitative measure thereof. The striae andinterference methods as combined in the present invention will behereinafter described in greater detail.

In the physics of flow there is an ever increasing demand for renderingvisible and exactly measuring the phenomena of flow. Where nonstationaryshort-timed phenomena, such as, explosion waves, sound phenomena inliquids and gases, formation of whirls in flows and local rapgases andliquids around bodies of various kinds, are concerned, it is required tocarry out accurate measurements simultaneously with the qualitativeobservation. Of course, it is possible to carry out the striae methodand the interference method in time succession, but, this does notpermit the measurement of non-stationary phenomena.

The present invention has for its object to render possible thesimultaneous qualitativ and quantitative measurement by combinedapplication of both of the said methods.

In one special case (see Dr.-Ing. Theo Zobel Erhiihung derSchneidgeschwindigkeiten beim Brennschneiden durch neue Diisenformen VDIVerlag, Berlin, 1936) the interferenc method and the striae method havealready been used simultaneously for the investigation of supersonicflows. However, in this case radial-symmetrical flows were concerned thestructure of which could be investigated in the same manner byradioscoping in any direction perpendicularly to the axis of flow.

The invention, on the other hand, has for its object to render itpossible to investigate density fields of any shape, and moreparticularly plane fields of any kind and dimensions simultaneously, i.e., in the same phase by means of the two said methods, without a mutualdisturbance by the light rays of the two methods, and independently ofthe frequency of the phenomenon to b investigated.

According to the invention, this problem is solved in this manner that abundle of light rays of a striae method is combined with the bundle oflight rays of an interference method which passes through the densityfields of the test medium to be investigated, and that the two bundlespass through the medium to be investigated in the same direction,together and simul taneously, while two different images of thephenomenon are afterwards produced on an image screen.

The interference method and the striae meth od of Toepler as well as thecombined application of the two methods in accordance with the presentinvention will be better understood by reference to the followingdetailed description in connection with the accompanying drawing,showing schematically two arrangements for carrying out the inventionand in which:

Fig. 1 is a schematic view of an arrangement involving the use of lenseswhile Fig. 2 is a similar view of an arrangement idly changingphenomena, such as flowing of using concave mirrors instead of lenses.

Similar characters of reference denote similar parts in the two figures.

Referring now to the drawing in greater detail, and first to the partsof the arrangement used for carrying out the interference method, itwill be noted that the light radiated from a source of light I isconverted into a pencil of parallel rays by a lens 2, Fig. l, or by aconcave mirror 2, Fig. 2, respectively, and directed to an interferencesystem of four plates constituting plane mirrors 3, 4, 5, 6, of whichmirrors 3 and 5 are semi-opaque and light transmitting, and 4 and B areopaque. A heat filter 8 has been interposed as indicated to prevent theheat rays of the source of light I from passing through the arrangement.The plate 3 as stated is a semiopaque light-splitting mirror by whichthe pencil of rays is split up into two partial rays a and b of whichray b is reflected by mirrors 3 and 6 to the semi-opaque light-splittingmirror 5 which permits passag of this ray b to a screen 1, while the raya passes through the semi-opaque lightsplitting mirror 3 and isreflected by the mirrors 4 and 5 to the screen I where the two partialrays a and b are combined, through a lens 9, to form a combined image.

Provided the four plates or mirrors 3, 4, 5, 8 are parallel, and thedistances 345 and 3 6-5 are of the sam ength, it is possible to produceinterference strips on the screen I. The ray b is caused to pass throughthe test medium In constituting the density field to be investigated.Its optical path is changed therein in accordance with the refractionpower of the medium to be investigated. The change of the n interferencestrips thereby effected forms a direct measure for the change of thedensity in the medium to be investigated.

Now, according to the present invention, in order to render thephenomenon visible by a striae method, in the same bundle of rays, thebundle of rays coming from the source of light I4, is combined with theinterference ray, for example, with the aid of the semi-opaquelightsplitting plates l6 and I1. It is advantageous to use a lightsource l4 having a wave length different from that of source I, andtouse lightsplitting partly permeable plates l6 and H, for the sake of abetter yield of quantity of light. The interference ray 1) leaving I1 isthus reduced in its intensity to 25 per cent. In order to match theinterference ray to the same intensity, a glass plate l3 having atransmission power of 25 per cent is inserted in the path of the rays aas a compensator.

The partial ray 0 from light source I4 reflected from the plate I! isused for the striae method in the usual manner and produces a striaeimage 1c of the phenomenon on the image screen I, said imag having thesame siz as th interference image 1b by the side of it. The partial rayfrom source l4 passing through the plate I! is absorbed by a chromaticfilter 22 which is impermeable or non-transparent with respect to thewave length of light source l4. A corresponding filter 2| which absorbsthe remaining interference ray but permits passage of the stria ray maybe also inserted in the striae ray after its separation from theinterference ray, if desired.

By way of alternative, the parallel bundles of light required for thetwo aforesaid methods may be produced by concave mirrors 2' and I (Fig.2) or similar .means, instead of being produced -by lenses. In order toavoid a disturbing radiation of heat rays, a thermal filter as 8 or 19is inserted in front of each source of light.

It makes no difference which kind of interference method is ,used, butit is essential that the light ray of the striae method be combined withthe interference ray while passing through the medium to be examined.

The light path of the ray passing through this medium is thus changed,as compared to a ray passing through a medium of uniform density and, asa result, the interference strips are displaced or bent, the amount ofdisplacement or bending forming a direct measure for the density of themedium.

Referring now to the portion of the arrangement serving for the striaemethod of Toepler, illustrated in the same figures, it will be notedthat a source of light I4 is disposed in the focal point of a lens l5,Fig. 1, or of a concave mirror l5, Fig. 2, thereby producing a parallellight ray 0 which is projected through the mirrors I6 and I1 and a lensI8 upon the left hand portion 10 of screen 1.

Provided at the focal point between the lens 18 and the screen 1 is aunilateral stop member or notch 20 which is so adjusted as to justengage the light ray at the focal point if the same is unaffected by anyparticular density field.

Now, when the light ray is passed through the medium to be examined, thelight ray is deflected, as has been explained, depending upon therefracting properties of the medium present, whereby more or less of thelight ray is intercepted by the member 20 and, as a result, the image onscreen I is brightened or darkened. In this manner flaws in the mediumcan be rendered visible.

While I have so far described the two parts of my novel apparatus or thetwo parts of my novel method as separate parts, it should be noted thatthe two parts make up a structural combination or a single process,respectively, thus permitting the simultaneous examination of a commondensity field by the striae and interference methods producing twojuxtaposed images on the two portions lb and 1c of a common screen 1.

By means of the optical arrangement just described it is possible torepresent density fields of different kinds by both of the said opticalmethods at the same time and thereby to obtain an image of the processas well as the exact technical data for measurement of the same.

The apparatus may be constructed in this manner, that one partial ray ofthe two bundles of rays is used for the projection on a ground glass,while independently thereof the process may be recorded photographicallyor, in case of phenomena of a non-stationary character, it may berecorded cinematographically.

My novel method permits an accurate coordination of the results of theinterference and striae methods, even in case of short-timed phenomena.

My novel method is of a particular importance for the examination ofwings of aircrafts intended for high speed. The influence of thecompressibility of the air becomes more and more important with respectto the flowing of the air around the wing profile as the flying speed isincreased. As soon as the sound velocity is reached which in someinstances may occur already on the wings of the now existing types ofairplanes, at least in the form of local excessive speeds, theresistance and lift features and so the fundamental flyingcharacteristics of the airplane are undergoing a fundamental change. Insuch cases the profile of the wings must be shaped according toprinciples which are quite different from those applying for the fiyingspeeds-now used. My novel device therefore permits especially theexamination of high speed profiles and high speed flying bodies ofvarious kinds in high speed aerodynamic investigating channels.

By the application of the said combined optical method it is possible todetermine on the stationary wing, also during a change of the incidenceangle, by each photographic exposure, the image of the flowing process(striae image) as well as the density field (interference image) andthus to ascertain the distribution of pressure around the whole profileof the wing. This simultaneous recording of the process renders itpossible to recognize immediately the occurrence of local sound wavesand to arrive at conclusions regarding the required changes of theprofile, while the interference image obtained at the same time yieldsquantitative data regarding the change of lift produced by the soundwaves.

It will be noted, that in the field of compressible flow theconventional means for the measurement in the air flow, such as,nozzles, Prandtl tubes, Pitot tubes, thermometers and the like cannot beused because sound waves would occur atany measuring device introducedinto the fiow and the measurement would be disturbed in anuncontrollable manner.

My novel method not only permits examination of the phenomena directlyat the surface of the profile, as in the conventional measurements ofthe distribution of pressure by the provision of bores in the wing, butit is also possible to find out the conditions of the fiow at anydesired point in the surroundings of the profile whereby it is possibleto examine the phenomena in the boundary layer and the thicknessthereof.

Furthermore, all the essential phenomena of fiow on auxiliary devices,such as auxiliary front wings, disturbing fiaps, spreading fiaps andfowler wings and the like can be examined.

The method and apparatus of the present invention have been described indetail with reference to specific embodiments. It is to be understood,however, that the invention is not limited by such specific referencebut is broader in scope and capable of other embodiments than thosespecifically described and illustrated in the draw i I claim:

1. An'optical arrangement for the simultaneous quantitative andqualitative investigation of density fields of a test medium, comprisingfirst means for examining the density field by an interference method,said first means including means for producing a first pencil ofparallel light rays, means for splitting up said light pencil into twopartial light pencils respectively traversing two separate paths, one ofwhich paths comprises the test medium, an image screen, means forre-uniting said partial light pencils on said image screen after passagethrough said separate paths, means for causing interference of the twore-united partial light pencils, second means for examining the densityfield by a striae method, said second means including means forproducing a second pencil of parallel light rays, means forconcentrating said second light pencil in a focal point, a unilateraloptical stop membe! adapted to engage said second light pencil at itsfocal point, a second image screen, and

means for directing said second light pencil on said second screen,after passage thereof past said stop member, and third means forpermitting simultaneous investigation of a common density field by saidfirst and second means, said third means including means for combiningsaid second light pencil with the partial light pencil passing throughthe density field, and means for reseparating said second light pencilfrom said partial light pencil after common uni-directional passagethrough said density field, said two image screens being mounted inclose juxtaposition.

2. An optical arrangement for the simultaneous quantitative andqualitative investigation of density fields of a test medium, comprisinga first means for examining the density field by an interference method,said first means including means for producing a first pencil ofparallel light rays, means for splitting up said light pencil into twopartial light pencils respectively traversing two separate paths one ofwhich-paths comprises the test medium, an image screen, means forreuniting said partial light pencils on said image screen after passagethrough said separate paths, means for causing interference of the twore-united partial light pencils, a second means for examining thedensity field by a striae method, said second means including means forproducing a second pencil of parallel light rays of a difierent wavelength from said first light pencil, means for concentrating said secondlight pencil in a focal point, a unilateral optical stop member adaptedto engage said second light pencil at its focal point, a second imagescreen, and

means for directing said second light pencil on said second screen,after passage thereof past said stop member, and a third means forpermitting simultaneous investigation of a common density field by saidfirst and second means, said third means including means for combiningsaid second light pencil with the partial light pencil passing throughthe density field, means including a chromatic filter for re-separatingsaid second light pencil from said partial light pencil after commonuni-directional passage through said density field, said two imagescreen being mounted in close juxtaposition.

3. An optical arrangement for the simultaneous quantitative andqualitative investigation of density fields, comprising first means forexamining the density field by an interference method, said first meansincluding means for producing a first pencil of parallel light rays,means including a semi-opaque mirror for splitting up said light pencilinto two partial light pencils respectively traversing two separatepaths one of which paths comprises the test medium, an image screen,means including opaque mirrors and semi-opaque mirrors for re-unitingsaid partial light pencils on said image screen after passage throughsaid separate paths, means for causing interference of the two re-unitedpartial light pencils, second means for examining the density field by astriae method, said second means including means for producing a secondpencil of parallel light rays, means for concentrating said second lightpencil in a focal point, a unilateral optical stop member adapted toengage said second light pencil at its focal point, a second imagescreen, and means for directing said second light pencil on said secondscreen, after passage thereof past said stop member, and third means forpermitting simultaneous investigation of a common density field by saidfirst and second means, said third means including a semi-opaque mirrorfor combining said second light pencil with the partial light pencilpassing through the density field, and means for re-separating saidsecond light pencil from said partial light pencil after commonuni-directional passage through said density field, said two imagescreens being mounted in close juxtaposition.

4. An optical arrangement for the simultaneous quantitative andqualitative investigation of density fields. comprising first means forexamining the density field by an interference method, said first meansincluding means for producing a first pencil of parallel light rays,means including a half-permeable mirror for splitting up said lightlight pencil into two partial light pencils respectively traversing twoseparate paths one 01' which paths comprises the test medium, means inthe path of the other partial ray for comensating for the influence ofthe semi-opaque mirrors upon the light intensity of the light pencilpassing through the test medium, an image screen, means including opaquemirrors and semi-opaque mirrors for re-uniting said partial lightpencils on said image screen after passage through said separate paths,means for causing interference of the two re-united partial lightpencils, second means for examining the density field by a striaemethod, said second means in cluding means for producing a second pencilof parallel light rays, means for concentrating said second light pencilin a focal point, a unilateral optical stop member adapted to engagesaid second light pencil at its focal point, a second image screen, andmeans for directing said second light pencil on said second screen,after passage thereof past said stop member, and third means forpermitting simultaneous investigation of a common density field bysaidfirst and second means, said third means including a semi-opaque mirrorfor combining said second light pencil with the partial light pencilpassing through the density field, and means for reseparating saidsecond light pencil from said partial light pencil after commonuni-directional passage through said density field, said two imagescreens being mounted in close juxtaposition.

5. In a, method for the quantitative and qualitative investigation ofdensity fields of a test medium, the steps which comprise producing afirst pencil of parallel light rays, splitting up said light pencil intotwo partial light pencils, passing a testing one of said partial lightpencils through the test medium, re-uniting said partial light pencilsand causing interference thereof on an image screen after passagethrough separate paths of equal geometrical length, producing a secondpencil 01' parallel light rays, combining said second light pencil withthe testing partial light pencil, for common uni-directional passagethrough said test medium, re-separating said second light pencil fromsaid partial light pencil, concentrating the second light pencil in a.focal point and directing it past a unilateral optical stop memberengaging the light pencil at said focal point, and directing the secondlight pencil on said image screen to produce an image in closejuxtaposition to that of the first light pencil.

THEODOR ZOBEL.

